The Obama administration is warning of drastically expanded sanctions on Russia if a secession referendum in eastern Ukraine goes forward, and is signaling that Vladimir Putin's latest call for the vote to be postponed does not go far enough.

White House spokesman Josh Earnest said Wednesday that the referendum should be "canceled," not just postponed.

A day earlier, the top U.S. diplomat in Europe, Victoria Nuland, testified that the referendum could be a "trigger" for expanded sanctions. To the chagrin of senators frustrated at the modest pace of sanctions to date, though, she heavily qualified the threat.

She said expanded sanctions would be triggered if the referendum goes forward -- if Russia also recognizes the vote and sends in "peacekeepers."

"Oh, so not only do they have the vote, but ...Russians have to move in to eastern [Ukraine]?" Sen. John McCain, R-Ariz., said.

Still, Nuland said this is the kind of scenario that would prompt sanctions on broad sectors of the Russian economy, something the administration has threatened but not yet imposed.

"Obviously that would trigger sectoral sanctions," Nuland said.

On Wednesday, Putin reportedly called for Sunday's referendum to be postponed.

U.S. officials have voiced grave concern that the vote would try to replicate what happened in the Crimean Peninsula, where residents voted to secede -- and the Russian government promptly approved it and claimed Crimea as part of Russian territory despite international condemnation.

"This is the Crimea playbook all over again," Nuland said Tuesday.

Secretary of State John Kerry called the referendum "bogus."

Also Wednesday, Putin claimed that his military had pulled back from the Ukrainian border.

But White House and Pentagon officials said they have no evidence that is actually happening.

Pentagon spokesman Col. Steve Warren told Fox News the U.S. had "seen no change in Russian force posture along the Ukrainian border."

Putin made the remarks in a Moscow meeting with Swiss President Didier Burkhalter.

"We're always being told that our forces on the Ukrainian border are a concern. We have withdrawn them. Today they are not on the Ukrainian border, they are in places where they conduct their regular tasks on training grounds," Putin said, according to Reuters.